Although there are steps to preventing it next season, blight thrives on a situation we cannot control — the weather. Cool nights and warm days with average precipitation do the deed, exactly what we’ve had this summer.

When the 2009 growing season began, garden writers wrote about the coming tomato blight. The disease lived up to its press. It caused major problems from the Midwest to Maine.

Although there are steps to preventing it next season, blight thrives on a situation we cannot control — the weather. Cool nights and warm days with average precipitation do the deed, exactly what we’ve had this summer.

It is no surprise we checked our tomatoes and found the dreaded black spots on leaves. Once it takes hold, there’s no stopping it. Then it spreads to our tomato relatives, the eggplants, potatoes and peppers.

I took a quick survey of our gardeners. Everybody was talking about it. An Alliance, Ohio, grower’s 150 tomato plants were soon reduced to but six.

“It devastated our local crops,” she said. She is working with the Ohio State Extension Service on prevention.

That prevention begins this fall. Blight is a soil fungus that spreads in plant material. For this reason, the No. 1 prevention is to rotate your vegetable crops in a different part of the garden each year. Following a schedule for where crops are on a three-to-four-year rotation helps considerably.

We will be pulling tomato plants soon. The best plan is to bag them and put them out with the trash. Composting can spread disease to your heap and to other plants.

Here are other prevention tips:

Clean up: Viruses thrive in dead leaves and stems. Remove these as you see them. Weeds can be disease vectors and must be removed.

Boost immunity: All plants have an immune system against disease. The effectiveness depends on plant health, and that depends on good soil.

Be selective: Some tomato varieties are more resistant to disease than others. Use certified disease-free seed and plants. Note that some heirloom tomatoes are very susceptible.

Open up: Densely grown tomatoes are far more likely to suffer disease. The viruses are similar to human ones, meaning they are very contagious. Double or triple the recommended space between plants.

Watch the weather: Bight spreads when temperatures are below normal at night (50s) and reach the 70s during the day. Rain and heavy dew spread viruses. When these conditions occur, applying an anti-fungal spray every seven days may help.

The signs: Blight forms black lesions on leaves, followed by white mold. Spores are formed on the mold, which then are spread to other plants by water or wind.

TOMATO BLIGHTS

Early: Concentric rings and dark spots on leaves that soon turn yellow and wither.

Late: Affects leaves, stems and fruit; this is the disease that caused the Irish Potato Famine. Signs are greasy-looking leaves with gray or black spots followed by white mold that contains the contagion.

Note: Most tomato plants turn yellow at the end of the season, oldest leaves first. Yellow without the telltale spotting indicates an absence of disease.